DROSOPHILA ECDYSONE RECEPTOR MUTATIONS REVEAL FUNCTIONAL DIFFERENCES AMONG RECEPTOR ISOFORMS

The steroid hormone ecdysone directs Drosophila metamorphosis via thre e heterodimeric receptors that differ according to which of three ecdy sone receptor isoforms encoded by the EcR gene (EcR-A, EcR-B1, or EcR- B2) is activated by the orphan nuclear receptor USP. We have identifie d and molecularly mapped two classes of EcR mutations: those specific to EcR-B1 that uncouple metamorphosis, and embryonic-lethal mutations that map to common sequences encoding the DNA-and ligand-binding domai ns. In the larval salivary gland, loss of EcR-B1 results in loss of ac tivation of ecdysone-induced genes. Comparable transgenic expression o f EcR-B1, EcR-B2, and EcR-A in these mutant glands results, respective ly, in full, partial, and no repair of that loss.